Police questioning a teenager who was allegedly at the wheel of a car that killed a young child at a caravan site have been given an extra 12 hours to detain him.

A seven-year-old boy was knocked down and later died from his injuries in hospital after the accident at about 3pm on Saturday.

A 16-year-old boy was allegedly driving the BMW car involved in the accident at Shirenewton caravan site, east of Cardiff, south Wales. He was arrested by South Wales police on Saturday afternoon on suspicion of causing death by dangerous driving.

Earlier on Sunday police revealed that young children at the caravan site may be key witnesses to what happened and have urged their parents to come forward.

"Police investigating a road traffic collision on the Shirenewton Caravan Site in Cardiff have been granted a further 12 hours to question a 16-year-old boy arrested on suspicion of causing death by dangerous driving," South Wales police said on Sunday night. Officers said the car involved in the tragic collision was a red BMW 3 Series estate.

The seven-year-old victim, who has not been named, is described as being from the caravan site.